The X1 is excellent for the price. Kind of a zero brainer really. Honestly just get one ....... with a PC you can EQ out the bass to make it more competitive if that's your thong.
I was toying with getting a K712, but seeing as it appears to be a side grade, I'd far prefer to spend a bit more....... thing is: if I spend $600 fat one's on a gaming grade open headphone, basically there's not a lot to choose from. I don't want to go ape ... Open / $600 / good for games and musak: what's a guy to do?
Try to win these and hope you've got money left over for a Pro bias amp, perhaps? Or maybe this setup, if you've got a speaker amp on hand and shipping from Australia doesn't murder your budget. Or, just maybe, $600 will be just enough to get you an ESP/950 somewhere.
If anything, the headphones are the easy part. The amps, on the other hand...